The present invention relates generally to the elimination of false echoes produced by electrical noise, and more particularly to a method and apparatus for eliminating substantially false echoes produced by electrical noise induced into high gain receiver circuits.
High gain receiver circuits are used in many applications for the decoding of received echo signals. One such application by way of example is use of an ultrasonic signal for ranging or gauging. In such systems, an ultrasonic transducer is caused to transmit an ultrasonic signal towards a target area by a transmitter circuit and receive reflections or echoes from the target which are provided to a receiver circuit for the decoding thereof. The distance between the transducer and target may be determined by multiplying the round trip time between transmission of the ultrasonic signal and reception of its echo signal with the velocity of sound based on the medium through which the ultrasonic signal is traveling. The ultrasonic signal is generally a tone pulse, the tone of which being selected at a resonant frequency which may be easily discriminated from energy echoes received by the transducer from other sources, such as mechanical vibration or shock, for example. The tone pulses are also transmitted at a pulse repetition rate having an interpulse period based on the distance to be measured, i.e. the maximum round trip time of the tone pulse should fall well within the interpulse period.
Typical transducers include circuits for converting electrical signals to ultrasonic signals and vice versa. Generally, the electrical signals representative of the echoes received by the transducer are at low levels. Consequently, these electrical echo signals are amplified with a high gain amplifier which is generally included in the receiver circuit which becomes a high gain receiver circuit. These high gain amplifiers are susceptible to electrical noise from electromagnetic and other noise sources, including the transmitter. While the high gain receiver circuits generally include filter circuits to filter out induced electrical noise at frequencies removed from the ultrasonic resonant frequency, the filter circuits can not eliminate electrical noise within the bandwidth of the resonant frequency without decreasing or affecting the level of the echo signal being decoded by the receiver. Accordingly, undesirable noise interference which coincides with the resonant frequency bandwidth of an echo signal may be mistakenly decoded by the receiver as an echo signal or signals (false echoes) which will cause erroneous measurements to be taken. To compound matters, such electrical noise may be both synchronous and asynchronous with the pulse repetition rate of the received pulses.
The present invention which will be described in detail herein below overcomes the drawbacks of the current high gain receiver circuits and provides for the substantial elimination of false echoes produced by induced electrical noise which falls within the bandwidth of the resonant frequency of the received pulses to improve the resultant ranging measurements using the echo pulses, whether or not such electrical noise is synchronous or asynchronous to the pulse repetition rate of the received pulses.
In accordance with one aspect of the present invention, an ultrasonic ranging system comprises: an ultrasonic transducer; transmitter circuitry coupled to the transducer for causing the transducer to generate tone pulses of frequencies within a predetermined bandwidth and at a predetermined pulse repetition rate to a target area, the transducer for receiving echo tone pulses from the target area at times within interpulse periods of the predetermined pulse repetition rate and converting the received echo tone pulses into representative electrical echo pulses; receiver circuitry coupled to the transducer for receiving the electrical echo pulses from the transducer, the receiver circuitry susceptible to electrical noise within the frequency bandwidth of the electrical echo pulses to create false echo pulses; and a filter for eliminating substantially the false echo pulses by accepting only electrical echo pulses which occur synchronously among interpulse periods, wherein the accepted echo pulses are used for range measurements of the target area.
In accordance with another aspect of the present invention, a method of measuring range to a target area using ultrasonic pulsing comprises the steps of: generating ultrasonic tone pulses of frequencies within a predetermined bandwidth and at a predetermined pulse repetition rate to a target area; receiving the ultrasonic tone pulses from the target area at times within interpulse periods of the predetermined pulse repetition rate and converting the received echo tone pulses into representative electrical echo pulses; receiving the electrical echo pulses by receiver circuitry susceptible to electrical noise within the frequency bandwidth of the electrical echo pulses to create false echo pulses; eliminating substantially the false echo pulses by accepting only electrical echo pulses which occur synchronously among interpulse periods; and using the accepted echo pulses for range measurements of the target area.
In accordance with a further aspect of the present invention for use with a high gain receiver circuit which receives echo pulses of frequencies within a predetermined bandwidth and at times within interpulse periods of a predetermined pulse repetition rate and amplifies the echo pulses for measurement purposes, the receiver circuit susceptible to electrical noise within the frequency bandwidth of the received echo pulses to create false echo pulses, a method of eliminating substantially the false echo pulses without affecting substantially the amplified echo pulses comprises the step of eliminating substantially the false echo pulses by accepting only echo pulses which occur synchronously among interpulse periods.